Revert "Simplify friends caching by only doing this for root agents - no functions require caching for child agents."

We need to cache child agents so that friends object edit/delete permissions will work across boarders on regions hosted by different simulators.

This reverts commit d9f7b8549b.

OpenSim/Framework/IClientAPI.cs

@@ -710,7 +710,7 @@ namespace OpenSim.Framework
         /// The scene agent for this client.  This will only be set if the client has an agent in a scene (i.e. if it
         /// is connected).
         /// </summary>
-        ISceneAgent SceneAgent { get; set; }
+        ISceneAgent SceneAgent { get; }
 
         UUID SessionId { get; }